Compiling and Linking

When programmers talk about creating programs, they often say, "it compiles
fine" or, when asked if the program works, "let's compile it and see". This
colloquial usage might later be a source of confusion for new programmers.
Compiling isn't quite the same as creating an executable file! Instead,
creating an executable is a multistage process divided into two components:
compilation and linking. In reality, even if a program "compiles
fine" it might not actually work because of errors during the linking phase. The total process of going from source code files to an executable might better be referred to as a build.

Compilation

Compilation refers to the processing of source code files (.c, .cc, or
.cpp) and the creation of an 'object' file. This step doesn't create anything
the user can actually run. Instead, the compiler merely produces the machine
language instructions that correspond to the source code file that was
compiled. For instance, if you compile (but don't link) three separate files,
you will have three object files created as output, each with the name
<filename>.o or <filename>.obj (the extension will depend
on your compiler). Each of these files contains a translation of your source
code file into a machine language file -- but you can't run them yet! You need
to turn them into executables your operating system can use. That's where the
linker comes in.

Linking

Linking refers to the creation of a single executable file from multiple
object files. In this step, it is common that the linker will complain about
undefined functions (commonly, main itself). During compilation, if the
compiler could not find the definition for a particular function, it would just
assume that the function was defined in another file. If this isn't the case,
there's no way the compiler would know -- it doesn't look at the contents of
more than one file at a time. The linker, on the other hand, may look at
multiple files and try to find references for the functions that weren't
mentioned.

You might ask why there are separate compilation and linking steps. First,
it's probably easier to implement things that way. The compiler does its
thing, and the linker does its thing -- by keeping the functions separate, the
complexity of the program is reduced. Another (more obvious) advantage is that
this allows the creation of large programs without having to redo the
compilation step every time a file is changed. Instead, using so called
"conditional compilation", it is necessary to compile only those source files
that have changed; for the rest, the object files are sufficient input for the
linker. Finally, this makes it simple to implement libraries of pre-compiled
code: just create object files and link them just like any other object file.
(The fact that each file is compiled separately from information contained in
other files, incidentally, is called the "separate compilation model".)

To get the full benefits of condition compilation, it's probably easier to get
a program to help you than to try and remember which files you've changed since
you last compiled. (You could, of course, just recompile every file that has a
timestamp greater than the timestamp of the corresponding object file.) If
you're working with an integrated development environment (IDE) it may already
take care of this for you. If you're using command line tools, there's a nifty
utility called make that
comes with most *nix distributions. Along with conditional compilation, it has
several other nice features for programming, such as allowing different
compilations of your program -- for instance, if you have a version producing
verbose output for debugging.

Knowing the difference between the compilation phase and the link phase can
make it easier to hunt for bugs. Compiler errors are usually syntactic in
nature -- a missing semicolon, an extra parenthesis. Linking errors usually
have to do with missing or multiple definitions. If you get an error that a
function or variable is defined multiple times from the linker, that's a good
indication that the error is that two of your source code files have the same
function or variable.